The part(s) or condition(s) listed below for the symptom Furnace not heating are ordered from most likely to least likely to occur. Check or test each item, starting with the items at the top of the page.
Furnace Blower Not Running? Gas Furnace Troubleshooting
Furnace Blower Runs All The Time? Gas Furnace Troubleshooting
Furnace Noisy Or Loud? Gas Furnace Troubleshooting
Furnace Starts Then Stops? Gas Furnace Troubleshooting
Furnace Not Heating? Gas Furnace Troubleshooting
Replace Furnace Filters - Tech Tips from Repair ClinicThe igniter gets very hot and glows bright orange to light the gas burner. If the igniter fails or cracks, the furnace won’t heat. To determine if the igniter is faulty, remove the igniter and inspect it for cracks. If the igniter is cracked, replace it. If the igniter is not cracked, use a multimeter to test the igniter for continuity. If the igniter does not have continuity, replace it.
Check the gas control valve to ensure that it is turned on and there is gas present at the valve. Natural gas pressure should be close to 4 inches water column at the furnace input; propane pressure close to 11 inches water column at the furnace input. The gas pressure can be checked with a manometer at the valve.
Check to see if there is power to the furnace. There should be a steady light on the control board. If there is no power to the furnace, check the home's furnace circuit breaker or fuse. Confirm the power switch on the side of the furnace is turned on.
The flame sensor monitors the burner to detect whether or not a flame is present. If the flame sensor is defective, it might fail to detect a flame. If the flame sensor does not detect a flame, the control board will shut off voltage to the gas valve to prevent the furnace from heating. Sometimes, if the flame sensor is dirty, it will fail to detect a flame. Try cleaning the flame sensor with a fine abrasive pad. If the furnace still doesn’t heat, replace the flame sensor.
If the furnace stops working properly the control board will often display a fault code to help diagnose the problem. The fault codes are displayed by a flashing light or multiple lights on the control board. Each sequence of flashes followed by a pause will indicate either an error code or the status of the furnace. Some manufacturers will use red and green lights to indicate specific fault codes. Fault codes are often displayed on the access cover of the furnace for easy reference.
The draft inducer fan motor assembly creates a vacuum when running which closes the pressure switch. Check the pressure hose and port on the inducer assembly for leaks or obstructions. A clogged condensate trap or drain, or restricted heat exchanger, can also cause a pressure switch to not function. Replace the pressure switch if there is proper vacuum or pressure but the switch is not working properly. You can test the pressure switch for electrical continuity with a multimeter. The contacts will either close or open depending on the application.
The spark igniter or electrode is the component that pulses to light the gas burner. If the spark igniter or electrode fails, the furnace won’t heat. Inspect the component for damage and replace if necessary.
The draft inducer motor draws air into the heat exchanger and then exhausts it out the flue. The pressure switch senses a pressure change and closes a switch to signal the control board that the furnace has proper air flow. If the draft inducer motor is defective, it may be unable to close the pressure switch, causing the ignition process to stall and the furnace to shut off after a few minutes. If the ignition process stalls, the furnace will shut off. If the draft inducer motor is at fault, replace it.
Most wall thermostats are either powered by an enclosed battery or by the furnace transformer. If the thermostat display is dead, you can remove the thermostat cover and look for replaceable batteries. Replace the batteries if needed and check operation.
The control board regulates the power supply to all of the components of the furnace. If the control board fails, it might not send voltage to the ignition system, causing the furnace not to heat.
The flame rollout switch monitors the heat surrounding the burners. If the inducer fan motor cannot draw enough air through the burners, or if the furnace is not venting properly, the rollout switch will open to halt the ignition sequence and prevent the furnace from heating. To determine if the flame rollout switch is defective, use a multimeter to test it for continuity.If the switch does not have continuity and the reset button is not tripped replace it. Be aware that poor airflow through the burner is often a result of insufficient ventilation or a restricted exhaust vent. Make sure that your furnace is in a properly ventilated location and the exhaust vent is clear of obstructions.
The wall thermostat has electrical contacts that control the power supply to the furnace. If the contacts in the thermostat fail, the furnace will not turn on. To determine if the thermostat is defective, use a multimeter to test it for continuity. If the wall thermostat does not have continuity, replace it.
The gas valve opens to allow gas to flow into the burner. If the gas valve is defective, the valve will not open, preventing the furnace from heating. To determine if the gas valve is at fault, use a multimeter to test it for continuity. If the gas valve does not have continuity, replace it.
The pressure switch closes when the burner has sufficient airflow. If the pressure switch isn’t working, it might not close when the proper airflow is present, causing the ignition process to stall. To determine if the pressure switch is defective, use a multimeter to test the switch for continuity. If the pressure switch does not have continuity, replace it.
The furnace must have proper airflow through the burner in order to run properly. The furnace draws air from the surrounding area or through tubing from outside the home or building. If the furnace is located in a utility closet or in a cramped storage room, there may not be sufficient available air for the furnace to heat properly. If the furnace draws air from outside a home or building, check for obstructions at the air inlet and exhaust outlet pipes.
The spinning blower wheel will circulate the heated air throughout the home's venting. If something is obstructing the blower wheel and preventing it from turning, the air won't be circulated. Inspect the blower wheel and housing for any obstructions and confirm the wheel is properly seated on the motor shaft and can rotate freely.
The control module regulates the power supply to all of the components of the furnace. If the control module fails, it might not send voltage to the ignition system, causing the furnace not to heat.
Electric furnaces will use a heating element instead of a burner assembly to heat the air that will be circulated through the home. A defective heating element can cause the furnace not to heat. Most electric elements require 220 volts and there may be more than one element. You use a multimeter to test all the elements and limit thermostats for electrical continuity, a continuous electrical path present in the components, to help determine if they are defective. The heating element may come as an assembly with limit thermostats included.
The blower motor spins a blower wheel to circulate the heated air throughout the home's venting. If the motor is burned out, the air will not be circulated. You should try turning the blower wheel by hand to determine if it can rotate freely. If blower wheel does not turn freely, it's likely the blower motor bearings have seized and the motor will need to be replaced. You can also use a multimeter to determine if voltage is reaching the motor. If power is getting to the component, but the motor doesn't run, it is likely defective.
The ignition module is the component that causes a spark igniter to spark and sends power to the gas valve in order for the burners to light. If the ignition module fails, the burners will not light and the furnace will not heat.
A furnace transformer is the component that decreases or increases the incoming voltage to power the unit. A faulty transformer can prevent the furnace from heating. If there is no secondary 24 volt power to the thermostat, the furnace trips a circuit breaker when it comes on, or you hear a consistent humming or vibrating noise, it's possible the transformer has failed.
The furnace thermostat monitors the heat being generated by the furnace and will shut off power to the heating circuit if the temperature gets too high. A defective thermostat may not allow the furnace to heat the air at all. You can use a multimeter to test the thermostat for electrical continuity, a continuous electrical path present in the component. If the thermostat tests negative for continuity, the part should be replaced.
Main Control Board
The furnace control board sends a low voltage signal to the contactor. When the contactor receives the signal from the control board, it closes the circuit, sending voltage to the other heating circuit components. If the contactor is defective, the heating circuit will not receive the voltage needed to operate. To determine if the contactor is at fault, use a multimeter to test the contactor for continuity. If the contactor does not have continuity, replace it.
The draft inducer motor assembly draws air into the heat exchanger and then exhausts it out the flue. The pressure switch senses a pressure change and closes a switch to signal the control board that the furnace has proper air flow. If the draft inducer motor is defective, it may be unable to close the pressure switch, causing the ignition process to stall and the furnace to shut off after a few minutes. If the ignition process stalls, the furnace will shut off. If the draft inducer motor assembly is at fault, replace it.
The WiFi thermostat has electrical contacts that control the power supply to the furnace. If the contacts in the thermostat fail, the furnace will not turn on. To determine if the thermostat is defective, use a multimeter to test it for continuity. If the WiFi thermostat does not have continuity, replace it.
The flame rollout or limit switch monitors the heat surrounding the burners. If the inducer fan motor cannot draw enough air through the burners, or if the furnace is not venting properly, the rollout limit switch will open to halt the ignition sequence and prevent the furnace from heating. To determine if the flame rollout limit switch is defective, use a multimeter to test it for continuity. If the switch does not have continuity and the reset button is not tripped replace it. Be aware that poor airflow through the burner is often a result of insufficient ventilation or a restricted exhaust vent. Make sure that your furnace is in a properly ventilated location and the exhaust vent is clear of obstructions.
